Methods for preparing and testing a thermal-spray coated substrate

ABSTRACT

A method for fabricating and testing an article having a thermal-spray coating thereon. The method includes providing a substrate article having a surface, thermally spraying a coating material onto the surface of the substrate article, wherein a surface of contact between the coating material and the substrate article is a bondline, and nondestructively testing the coated article. Nondestructively testing includes generating an eddy current in the coated article, measuring the eddy current in the coated article, and evaluating a near-bondline region of the coated article located adjacent to the bondline using the measured eddy current.

BACKGROUND OF THE INVENTION

This invention relates generally to the preparation of substrates havinga thermal-spray coating thereon, and more particularly to methods oftesting of the integrity of a bond between the thermal-spray coating andthe substrate.

At least some known metallic coatings are applied to substrates using athermal-spray process in which a coating material, usually provided in apowder or wire form, is heated to an elevated temperature in a spraydevice. The coating material may be fully melted to form liquiddroplets, may be partially melted to form semiplastic particles, or mayremain formed as solid powder particles. The coating material isdischarged from the spray device at a high velocity and towards asubstrate surface. The sprayed material is deposited on the surface and,to the extent that it is liquid, solidifies. More specifically, dropletsand particles impact the surface at a relatively high velocity, and aresubstantially flattened against the surface. The deposition continuesuntil the solidified coating has reached a desired thickness.

The thermal-spray process is highly versatile and may be used with awide variety of compositions and substrate articles. For example, thethermal-spray process may be used to deposit a coating on an articlethat has been partially worn away during prior service, wherein thecoating has substantially the same composition as the substrate article.In another example, the thermal-spray process is used to deposit awear-resistant coating across a surface, wherein the coating has adifferent composition than the substrate article and is morewear-resistant than the substrate article. In yet another example, thethermal-spray process may be used to deposit a wearing or abradablecoating across a surface, wherein the coating has a differentcomposition than the substrate article and is less wear resistant thanthe substrate article. Moreover, the thermal-spray process may be usedto coat irregular and complexly shaped article substrates.

Generally, to be effective, the thermally sprayed coating must adhere ata bondline to the entire surface to which it is applied with a goodmechanical bond. Accordingly, delaminations of the coating from thesubstrate may enable the coating to separate from the substrate. In somemore-demanding applications, the coating must further be metallurgicallybonded to the substrate.

At least one known method to determine the bonding strength of thebonding of the sprayed coating to the substrate requires destructivesectioning of the coated substrate article and metallurgical inspectionof the bondline region. This method is normally used to establishprocess parameters that achieve a good bonded coating, and then the sameprocess parameters are duplicated in the production coating operations.Because the thermal-spray process is so versatile, it may be difficultto perform destructive testing over the entire range of possible typesof coatings and configurations of substrate articles. Moreover, even ifa process is deemed through the destructive testing process, relativelyminor variations in production parameters may lead to unacceptablebondline structures in the production articles. Another problem with theuse of test articles is the test articles may behave differently thanthe production articles. Additionally, post-coating operations such asheat treating and machining may introduce bondline defects to initiallydefect-free bondlines.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a method for fabricating and testing an article having athermal-spray coating thereon is provided. The method includes providinga substrate article having a surface, thermally spraying a coatingmaterial onto the surface of the substrate article, wherein a surface ofcontact between the coating material and the substrate article is abondline, and nondestructively testing the coated article.Nondestructively testing includes generating an eddy current in thecoated article, measuring the eddy current in the coated article, andevaluating a near-bondline region of the coated article located adjacentto the bondline using the measured eddy current.

In another aspect, a system for testing an article having athermal-spray coating thereon is provided. The system includes aturntable having a thermally coated substrate article positionedthereon, an eddy current probe operatively coupled to the substratearticle, the eddy current probe configured to generate an eddy currentwithin the coated substrate article and measure the eddy current withinthe coated substrate article, and a computer configured to determine anear-bondline region of the coated article located adjacent to abondline using the measured eddy current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary gas turbine engine.

FIG. 2 is a perspective view of a turbine stationary seal used with thegas turbine engine shown in FIG. 1.

FIG. 3 is a block diagram of a system for testing an article having athermal-spray coating thereon.

FIG. 4 is an exploded view of the system shown in FIG. 3.

FIG. 5 is of a flowchart illustrating a method for testing an articlehaving a thermal-spray coating thereon.

FIG. 6 is a perspective view of an article having a thermal-spraycoating thereon.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration of a gas turbine engine 10 includinga fan assembly 12 and a core engine 13 including a high pressurecompressor 14, and a combustor 16. Engine 10 also includes a highpressure turbine 18, a low pressure turbine 20, and a booster 22. Fanassembly 12 includes an array of fan blades 24 extending radiallyoutward from a rotor disc 26. Engine 10 has an intake side 27 and anexhaust side 29. In one embodiment, the gas turbine engine is a CF6-50available from General Electric Company, Cincinnati, Ohio. Fan assembly12 and turbine 20 are coupled by a first rotor shaft 31, and compressor14 and turbine 18 are coupled by a second rotor shaft 33.

During operation, air flows axially through fan assembly 12, in adirection that is substantially parallel to a central axis 34 extendingthrough engine 10, and compressed air is supplied to high pressurecompressor 14. The highly compressed air is delivered to combustor 16.Airflow (not shown in FIG. 1) from combustor 16 drives turbines 18 and20, and turbine 20 drives fan assembly 12 by way of shaft 31.

FIG. 2 is a perspective view of an article 28 to be tested. In theexemplary embodiment, article 28 is a stationary seal 30 that includes aplurality of honeycomb land surfaces 32. For example, and in theexemplary embodiment, stationary seal 30 is a pressure balance seal thatcan be used with gas turbine engine 10 (shown in FIG. 1).

Although the methods herein are described with respect to a stationaryseal, it should be appreciated that the methods can be applied to a widevariety of articles. For example, article 28 may be of any operableshape, size, and configuration. Examples of substrate articles ofinterest include areas of components of gas turbine engines such asseals and flanges, as well other types of articles. The substratearticle may be made of any operable base material. Examples of operablebase materials include nickel-base alloys, which have more nickel byweight than any other element; cobalt-base alloys, which have morecobalt by weight than any other element; titanium-base alloys, whichhave more titanium by weight than any other element; iron-base alloys,which have more iron by weight than any other element; and aluminum-basealloys, which have more aluminum by weight than any other element. Anexample of a nickel-base alloy of particular interest is Alloy 718,having a specification composition, in weight percent, of fromapproximately 50.0% to approximately 55.0% nickel, from approximately17.0% to approximately 21.0% chromium, from approximately 4.75.0% toapproximately 5.50% columbium plus tantalum, from approximately 2.8% toapproximately 3.3% molybdenum, from approximately 0.65% to approximately1.15% titanium, from approximately 0.20% to approximately 0.80%aluminum, approximately 1.0% percent cobalt, and a balance of irontotaling 100% by weight. Small amounts of other elements such as carbon,manganese, silicon, phosphorus, sulfur, boron, copper, lead, bismuth,and selenium may also be present. These substrate articles andcompositions are presented by way of examples of preferred embodiments,and not by way of limitation.

In one embodiment, coating 40 is a thermal barrier coating such as, butnot limited to, a Nickel Chromium Aluminum (NiCrAl) coating having anominal composition range, in weight percent, of from approximately 4.5%to approximately 7.5% percent aluminum, from approximately 15.5% toapproximately 20.5% chromium, approximately 3.0% manganese,approximately 1.0% iron, approximately 0.3% carbon, approximately 2.0%silicon, approximately 3.5% of other elements, and approximately 70.0%nickel. In the exemplary embodiment, coating 40 is between approximately0.002 inch and approximately 0.150 inch in thickness and may be appliedto stationary seal 30 using a quantity of thermal spray techniques suchas, but not limited to, high velocity oxyfuel spray (HVOF), air plasmaspray (APS), low-pressure-plasma spray (LPPS), electric wire arc spray,and combustion wire or powder spray. After coating 40 is applied to asurface 64 of article 28, a heat treatment operation is performed tofacilitate diffusing coating 40 into article 28. Coating 40 is thennon-destructively tested to determine if any bondline faults existbetween coating 40 and surface 64 of article 28. More specifically, awide variety of factors, such as the shape of article 28, i.e.stationary seal 30, the base material of article 28, the coatingmaterial, i.e. coating 40, and variations in operating parameters mayresult in near-bondline flaws between article 28 and coating 40. Suchflaws may cause thermal-spray coating 40 to perform in an unsatisfactorymanner. Therefore article 28 is tested to determine whether such flawsare present in article 28 and when coated substrate article 28 is freeof such flaws.

FIG. 3 is a system 50 that may be used to non-destructively test abondline between a metallic substrate, such as stationary seal 30, and acoating applied to the substrate, such as coating 40. FIG. 4 is aportion of system 50 shown in FIG. 3. In the exemplary embodiment,system 50 is an eddy current inspection system 50 that includes a dataacquisition/control system 52, and an eddy current probe 54 having a cam56. In the exemplary embodiment, eddy current probe 54 is a cam followerprobe configured to operate at approximately 500 kiloHertz. Eddy currentprobe 54 is electrically coupled to data acquisition/control system 52such that control/data information can be transmitted to/from eddycurrent probe 54 and data acquisition/control system 52. System 50 alsoincludes a turntable 58 configured to rotate around an axis 60, and amechanical member 62 such as, but not limited to, a robotic arm slidablycoupled to article 28 such as, a portion of turbine stationary seal 30positioned on turntable 58.

FIG. 4 is an enlarged view of data acquisition/control system 52 andeddy current probe 54 shown in FIG. 3. Although eddy current probe 54 isshown as a two-dimensional sensor array similar to that disclosed inU.S. Patent Publication No. US 2002/0190724 A1 of Ser. No. 09/681,824,filed Jun. 12, 2001 and assigned to General Electric Company,configurations of the present invention do not necessarily requireeither a two-dimensional sensor array or the two-dimensionalcapabilities disclosed in that Patent Publication.

Eddy current probe 54 includes a drive coil 70, which is shown partiallycut away in FIG. 4 to reveal more details of an included sensor orsensors 72, and a square pulse generator 74. Data acquisition/controlsystem 52 includes a computer interface 76, a computer 78, such as apersonal computer with a memory 80, and a monitor 82. Computer 78executes instructions stored in firmware (not shown). Computer 78 isprogrammed to perform functions described herein, and as used herein,the term computer is not limited to just those integrated circuitsreferred to in the art as computers, but broadly refers to computers,processors, microcontrollers, microcomputers, programmable logiccontrollers, application specific integrated circuits, and otherprogrammable circuits, and these terms are used interchangeably herein.

Drive coil 70 is a multiple turn solenoid that can be of generallyrectangular configuration surrounding sensor or sensors 72. Sensors 72can be located inside or outside as well as above or below drive coil70. Rectangular drive coil 70 is used to transmit a transientelectromagnetic flux into a metallic object under test such as article28 (shown in FIG. 3). Memory 80 is intended to represent one or morevolatile and/or nonvolatile storage facilities not shown separately thatare familiar to those skilled in the art. Examples of such storagefacilities often used with computer 78 include solid state memory (e.g.,random access memory (RAM), read-only memory (ROM), and flash memory),magnetic storage devices (e.g., floppy disks and hard disks), opticalstorage devices (e.g., CD-ROM, CD-RW, and DVD), and so forth. Memory 80may be internal to or external to computer 78. Data acquisition/controlsystem 52 also includes a recording device 84 such as, but not limitedto, a strip chart recorder, a C-scan, and an electronic recorder,electrically coupled to at least one of computer 78 and eddy currentprobe 54.

FIG. 5 is a method 100 for fabricating and testing an article having athermal-spray coating thereon. Method 100 includes providing 102 asubstrate article 28 having a surface 64, and thermally spraying 104 acoating material 40 onto surface 64 of substrate article 28, wherein asurface of contact between coating material 40 and substrate article 28is defined as a bondline. Coated substrate article 28 is then positionedon turntable 58. Turntable 58 is then energized such that coatedsubstrate article 28 is rotated around axis 60 of turntable 58. Method100 further includes nondestructively testing 106 coated substratearticle 28, wherein nondestructively testing 106 includes generating 108an eddy current in coated substrate article 28, measuring 110 the eddycurrent in coated substrate article 28, and evaluating 112 anear-bondline region of coated substrate article 28 located adjacent tothe bondline using the measured eddy current.

More specifically, pulse generator 74 is used to excite drive coil 70with an essentially rectangular-shaped short duration pulse ofelectrical current while sensors 72 and coil 70 are on or proximatesurface 64 of coated substrate article 28. As a result, a pulsed eddycurrent is generated in coated substrate article 28 under test. Sensoror sensors 72 sense the pulsed eddy current as a voltage. For example,the pulsed eddy current may produce a signal ranging from approximately+500 mV to approximately −500 mV in sensor or sensors 72 for aparticular article 28. In the exemplary embodiment, only a signalgenerated by one sensor 72 is considered for the remainder of thisdiscussion, as a plurality of sensors 72 is not required to practicemany configurations of the present invention. Also, sensor 72 mayproduce either a voltage or a current indicative of the pulsed eddycurrent. Therefore, “a measured eddy current,” as used herein, includesany measured representation of the eddy current, whether therepresentation is in the form of a voltage, a current, or a digitizedvalue.

Computer interface 76 receives a response signal from sensor 72 andcommunicates a digitized signal representative of the pulsed eddycurrent during a measurement window into computer 78. In the exemplaryembodiment, the measurement window commences very shortly after thepulse ends. For example, in some configurations, the measurement windowbegins approximately 10 ms after the pulse ends. In otherconfigurations, the measurement window begins approximately 0.5 ms afterthe pulse ends. Utilizing a stored program in memory 80, computer 78parameterizes this digitized signal and applies a transfer function tothe parameters to determine at least one measurement/object property. Asused herein, a “measurement/object property” is a physical property ofthe metallic object itself, such as wall thickness, permeability, orconductivity, and/or a property of the measurement, i.e., a physicalrelationship between the metallic object and the sensor, such as sensorliftoff. A result is then displayed on display 82 and/or saved in memory80 and/or printed on a printer (not shown in the figures) for later use.In another embodiment, the digitized signal is received at recordingdevice 84 and stored for later use.

The received signal is then evaluated 112 to determine whether anear-bondline region 66 is delaminated, exhibits a mechanical bond (withno delamination), or exhibits a metallurgical bond (with nodelamination). As used herein, near-bondline region 66 includes, but isnot limited to, a flat bottom hole for example, in coating 40.Evaluating 112 includes generating a preferred acceptability criterion.Specifically, and referring to FIG. 6, article 28 is dynamically testedusing system 50 as described previously herein. In operation, system 50is capable of determining a bondline fault region 66 that includes asubstantially flat bottom 90 that is approximately 0.020 inches in depth92, wherein the fault region is approximately 1/32 of an inch in width94. At least one of computer 78 and recording device 84 is then used todetermine whether bondline fault region 66 is within acceptable limits.Any bondline fault region which exceeds the predetermined threshold isthen evaluated to determine an actual size.

The above-described methods and system provide a cost-effective andreliable means for facilitating determining near bondline faults inthermal spray coated articles. Although the methods are described withrespect to coating and testing an object that includes an approximatelycylindrical outer surface, it should be realized that the methods can beused for an article having a complex outer surface. For example, adigital eddy current proximity system may be used to measure the sizeand depth of a near bondline fault in a turbine seal. The methodsdescribed herein may also be used both as a process-development tool todetermine the required processing of the thermally sprayed article, andas an acceptance test on production hardware to determine itsacceptability. Additionally, using a cam-follower probe that is mountedon a robotic arm facilitates measuring the eddy current automatically,since the cam follower probe is configured to follow any contourautomatically, thus enabling testing a wide variety of substratearticles.

Exemplary embodiments of digital eddy current proximity systems aredescribed above in detail. The systems are not limited to the specificembodiments described herein, but rather, components of each system maybe utilized independently and separately from other components describedherein. Each system component can also be used in combination with othersystem components.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A system for use in testing an article having a thermal-spray coatingthereon, said system comprising: a turntable having a thermally-coatedsubstrate article positioned thereon; an eddy current probe operativelycoupled to said substrate article, said eddy current probe configured togenerate an eddy current within said coated substrate article and tomeasure the eddy current within said coated substrate article; and aprocessor configured to determine a near-bondline region of said coatedarticle located adjacent to a bondline using the measured eddy current.2. A system in accordance with claim 1 wherein said eddy current probecomprises a cam follower probe configured to translate along an outerperiphery of said coated substrate article; and to generate an eddycurrent within said coated substrate article.
 3. A system in accordancewith claim 2 further comprising a robotic arm coupled to said camfollower probe, said robotic arm configured to receive instructions froma computer and to translate said cam follower probe along an outerperiphery of said coated substrate article in accordance with saidreceived instruction.
 4. A system in accordance with claim 1 whereinsaid eddy current probe comprises: a drive coil; a pulse generatoroperable to energize said drive coil in a pulsed manner to transmit atransient electromagnetic flux to into a metallic object underinspection; and at least one sensor operable to generate output signalsrepresentative of time varying eddy currents produced in said coatedarticle substrate from said transient electromagnetic flux.
 5. A systemin accordance with claim 4 wherein said at least one sensor isconfigured to determine a near bond-line fault that is less thanapproximately 0.03125 inches in depth, and less than approximately 0.020inches in width.
 6. A system in accordance with claim 4 furthercomprising a processor coupled to said at least one sensor andconfigured to: measure the output signals representative of thetime-varying eddy currents resulting from said transient electromagneticflux; determine whether measured output signals exceed a predeterminedthreshold.
 7. A system in accordance with claim 1 further comprising adata acquisition/control system configured to record an output receivedfrom said eddy current probe.
 8. A system in accordance with claim 1wherein said turntable is configured to rotate while said eddy currentprobe is generating an eddy current within said coated substratearticle.
 9. A system in accordance with claim 1 wherein said coatedsubstrate article comprises a gas turbine engine stationary seal.
 10. Asystem in accordance with claim 9 wherein said gas turbine enginestationary seal comprises a metallic material thermally sprayed onto asurface of said stationary seal.